1. Field of the Invention
The present invention relates to a light emitting device, and particularly to a light emitting device having high heat resistance and high durability.
2. Description of the Related Art
Among light emitting devices that employ light emitting elements such as light emitting diode, a light emitting device having a housing which protects the to the light emitting element is known. The housing has a recess formed on the light emitting side thereof so that, after the light emitting element is mounted in the recess, the recess is filled with a transparent sealing material for the protection of the light emitting element from the environment.
The housing is formed from an electrically insulating resin which is appropriate for electrically and mechanically protecting the light emitting element. For example, a nylon-based resin such as polyphthalamide or polyamide, a liquid crystal polymer, or an epoxy-based resin is preferably used.
For the sealing material, such a material is used that has translucency and good bonding with the housing, and an epoxy resin is commonly used. While the epoxy resin is known to undergo deterioration and discoloration to become yellowish when exposed to light and/or heat over an extended period of time, this problem may be negligible if used in a light emitting device of which intensity of light emission is not high and heat generation is low.
However, as light emitting devices are manufactured with increasingly higher output power, the epoxy resin used as the sealing material undergoes more significant yellowing, which results in a decrease in the light transmittance of the epoxy resin. As a result, efficiency of extracting light from the light emitting device decreases. In the case of a light emitting device which emits light of shorter wavelength, in particular, yellowing due to the deterioration and decrease in the light transmittance become substantial. While service life of the light emitting device has been increasing, there has been such a problem that the epoxy resin has lower durability than the other components of the light emitting device.
Accordingly, it has been proposed to use a silicone resin, which is transparent and is less vulnerable to deterioration due to light and heat, instead of the epoxy resin as the sealing material (for example, refer to Kohyo (National Publication of Translated Version) No. 2006-516816). The silicone resin has siloxane bond of high bonding energy in the backbone of base polymer, and is one of resins which have high heat resistance and high weatherability. The silicone resin also has high light transmittance, and is regarded as a promising sealing material to be used in light emitting devices of high output power.
However, the silicone resin which has high translucency and high durability also as a drawback of low bonding with the housing. That is, when the silicone resin is used as the sealing material, it may peel off first in the inner edge of opening of the recess and then in the inner portion of the recess, due to the mechanical and thermal stresses caused between the housing and the sealing material. In the worst case, the sealing resin may completely fall off the housing.
As a technique to solve this problem, it is known to provide a wall around the recess of the housing and a ring-shaped trench between the wall and the inner edge of the opening (for example, refer to Kohyo (National Publication of Translated Version) No. 2006-516816). The sealing material is provided to fill not only the recess of the housing but also the trench, so as to serve as an anchor ring. In this housing, the wall is formed so that the top of the wall is lower than the light emitting surface of the housing when viewed from the bottom of the recess.
Peel-off between the inner edge of the opening of the recess of the housing and the sealing material may lead to the infiltration of moisture and impurities into the light emitting device, as well as to causing the separation of the sealing material to start. When moisture or other impurity which has infiltrated through a peeling gap deposits on the inner surface of the recess, reflectivity of the housing decreases which results in decreasing efficiency of extracting light from the light emitting device. When the impurity reaches the light emitting element, the light emitting device may fail to function. The technology disclosed in Kohyo (National Publication of Translated Version) No. 2006-516816 may suppress the sealing member from peeling off in the interface with the housing and falling off, although it is unable to prevent peel-off on the inner edge of the opening of the recess of the housing.
In case the silicone resin is used for the sealing resin, in particular, the problem of moisture or other impurity infiltrating through a peeling gap becomes serious, since the silicone resin has higher expansion coefficient than the housing. As the temperature of the light emitting device rises during operation, the silicone resin expands more than the housing and swells outwardly from the inner edge of the opening of the recess of the housing. In case there is a gap between the sealing resin and the inner edge of the opening of the recess of the housing due to peeling, the sealing resin bulges out of the housing as the temperature of the light emitting device rises. Then, the sealing resin returns within the housing as the temperature decreases. As this thermal cycle is repeated, bonding between the sealing resin and the inner edge of the opening of the recess of the housing is lost, thus allowing impurities to enter through the gap.